How can I make a material for jam?
Watch out, you have an unconnected socket shader socket in your node tree.

Anyway maybe mix a little translucent shader into your glass one. You setup looks quite good to me already, much of the remaining work needed is probably pertaining to setting up a proper scene and environment. "Jam in the void" will never look like proper jam.

You might optionally also want to add a particle system to your jam object to simulate the seeds in suspension.